1. Field of the Invention
This invention relates to a fuel injector core and more particularly to a core accommodating a sleeve in the fuel injector.
2. Description of the Prior Art
Many of known fuel injectors have the construction as shown in FIG. 31. In the figure, reference numeral 201 indicates a cylindrical body. Installed in this body 201 are a cylindrical core 202 and a cylindrical armature 203, both of which are made of magnetic material. Formed at the middle of the core 202 is a flange 204 for magnetic path which is connected at its circumference with the body 201. The outer circumferential surface 205 of the retractable armature 203 is supported by the inner surface of the body 201 so that the armature 203 can be moved back and forth freely. Around the core 202 is arranged a coil 206 for generating a magnetic field. When the coil 206 is energized, the armature 203 retracts against the force of a coil spring 207 to allow fuel to be injected from a nozzle 208. The rear end 209 of the coil spring 207 is supported on a sleeve 211 installed inside a sleeve insertion hole 210 of the core 202.
If the force of the coil spring 207 in the fuel injector 200 is too strong, the operation timing of the armature 203 is delayed. When it is too weak, the armature operation timing becomes early. Any delay or advance in the armature operation timing will result in the amount of injected fuel becoming inappropriate.
To avoid this problem requires the spring 207 to be adjusted at a proper spring force. For this purpose, when the sleeve 211 is inserted into the insertion hole 210, a measuring device (not shown) is used to measure a repulsive force the sleeve 211 receives from the spring 207. As the amount of insertion of the sleeve 211 gradually increases, so also the repulsive force increases. When a proper repulsive force is obtained, insertion of the sleeve 211 is stopped and at this position the sleeve 211 is securely and correctly fixed to the core 202 without any deviation.
If, however, there is a frictional resistance against the sleeve insertion between the outer circumferential surface of the sleeve 211 and the inner circumferential surface of the insertion hole 210, an error will occur in the measured value of the repulsive force of the spring 207. To prevent this error, the inner surface of the insertion hole 210 must be finished smooth. Another measure may be to enlarge the inner diameter of the insertion hole 210 with respect to the outer diameter of the sleeve 211. However, increasing these dimensions makes the above work--"where the insertion of the sleeve 211 is stopped the sleeve 211 is securely and correctly fixed to the core 202 without any deviation"--impossible. That is, when the gap between the core 202 and the sleeve 211 is large, the sleeve 211 will be slightly off-centered with respect to the core 202 when it is caulked to the core 202. This in turn shifts the relative position of the sleeve 211 with respect to the rear end of the spring 207, changing the force of the coil spring 207.
Under these circumstances, the conventional practice necessarily involves many processes in finishing the inner surface of the sleeve insertion hole 210. That is, the sleeve insertion hole 210 of the core is bored by a drill, finished by a reamer and then buff-finished. However, on the inner surface of the insertion hole 210 that is bored by a drill, fine scores on the order of micron remain even after it is subjected to a series of finishing processes. These minute scores have delicate effects on the back-and-forth sleeve movement, which in turn causes small errors in the measured values of the repulsive forces. Furthermore, with the minute scores remaining on the inner surface of the insertion hole 210 that was cut by a drill, chips the size of microns will eventually fall from the scores, preventing the normal operation of the valve.